The present invention relates in general to an alarm circuit and is concerned, more particularly, with a circuit for use in a vehicle to prevent the theft of the vehicle. The circuit of this invention is more particularly concerned with sensing a current flow in one of the battery cables connecting to the storage battery of the vehicle.
There presently exists two basic types of car alarms in which intrusion into the cab of the vehicle is detected by an increase in battery power required for the cab light. One alarm device detects an instantaneous change in current being drawn from the battery. The other device detects an instantaneous change in battery voltage. Both of these systems require initial adjustments and both of these systems are inherently unreliable. The current detection system does not always trip the alarm because of changes in parameters of the cab light circuit. The voltage sensing device is characterized by generating false alarm conditions especially in cold weather due to an uncontrollable intermittent internal drop in the battery voltage occasioned by a decrease in temperature. Accordingly, an object of the present invention is to provide an improved alarm circuit preferably for motor vehicles and which reliably detects a current being drawn by the storage battery of the vehicle.
Another object of the present invention is to provide an alarm circuit that is readily installed in a vehicle without the requirement of sensitive adjustments.
A further object of the present invention is to provide an alarm circuit for a vehicle that is simple in construction and is reliable without generating false alarm conditions.